The term “vehicle alternator” ordinarily refers to an alternating current generator for generating electricity by receiving rotation of a vehicle engine through a belt, and storing the thus generated electric power in a battery or supplying it to electric loads on the vehicle.
As shown in FIG. 2, such an alternator includes a rotary shaft 13 carrying a rotor 12 and rotatably supported by a housing through a pair of ball bearings A. The rotor 12 carries a rotor coil 15. A stator 16 is provided around the rotor 12 and includes three stator coils 17 arranged at angular intervals of 120°.
The rotary shaft 13 is rotated by the torque transmitted from the vehicle engine to a pulley 20 mounted on the tip of the rotary shaft 13 through a belt (not shown). Electric current thus flows through the rotor coil 15, which induces alternating currents in the respective three stator coils 17 in three phases. The thus induced three-phase alternating currents are full-wave rectified by a rectifier 21 and stored in a battery.
Since the pulley 20 is mounted to the rotary shaft 13 in a cantilever manner, and it vibrates when the rotary shaft 13 rotates at a high speed, especially the ball bearing A supporting the shaft 13 near the pulley 20 is subjected to severe loads. Numerals 18 and 19 in FIG. 2 indicate a step ring and a fan, respectively.
Since rolling bearings for use in alternators are rotated at high speed in heated engine room, they may be heated to 180° C. or over, and also may be rotated at 1000 rpm or over. Thus, they have to be capable of withstanding use in extremely harsh environments.
Such rolling bearings for use in alternators are required to be heat-resistant, and to be able to rotate at low torque. High durability is also required.
For this purpose, it is necessary to selectively use heat-resistant base oil and thickening agents in lubricating grease sealed in such rolling bearings. For example, it is known to use lubricating grease comprising a base oil in the form of a mixture of alkyl diphenyl ether oil and poly-α-olefin oil, and 5 to 40% by weight of an aromatic diurea compound as a thickening agent (Patent document 1).
There is also known a rolling bearing for use in electronic devices and air-conditioners in which lubricating grease having a dynamic viscosity of 30 to 70 cSt at 40° C. is sealed such that it accounts for 5 to 20% by volume of the entire inner void space of the bearing (Patent document 2).
Patent document 1: JP Patent Publication 5-263091A
Patent document 2: JP Patent Publication 2001-123190A